1. Field of the Invention
The present invention relates to assay methods and systems, and more particularly, to assay systems and methods using plasmonic emissions generated by approaching or aggregating metallic surfaces.
2. Background of the Related Art
Dynamic Light Scattering (DLS), also referred to as photon correlation spectroscopy,1 is the most widely used technique today for studying colloidal systems.2-5 It is a relatively fast technique, which can provide absolute estimates of particle size and concentration for a wide variety of particles. However, the technique does have several limitations.1 These limitations include the low information content from the measured signal, the complexity of data analysis (this involves the numerical inversion of a Laplace Transform1) and the fact that both DLS and the other scattering techniques are not appropriate for very dilute solutions of particles.2-5 Subsequently, this has been a limitation in sensing biospecies at nanomolar and even lower concentrations.
Over the last several years, the use of both gold and silver nanoparticles in biological assays has dramatically increased. This has been afforded by their very high molar absorption coefficients,6,7 which has enabled their use in many absorption-based (of light) nanoparticle assays.8-14 In addition to their high absorption cross-sections, nanoparticles of gold and silver are also very efficient scatterers of light. Indeed a noble metal colloid's extinction spectrum is composed of both an absorption and scattering component, which is contrary to how a typical fluorophores extinction spectrum is understood. Subsequently, light scattering by gold and silver nanoparticles can be detected at concentrations as low as 10−16 M6. In addition, it is well known that the light dependent scattering properties of a nanoparticle depend on their size, shape, composition and the refractive index of the suspending medium.6 
Typically, in cellular imaging today, fluorophores or even quantum dots, are used, which either contain some function groups to bind to expressed cellular surface features (receptors) or can even be transfected within the cells. This enables the cells to be readily imaged. However, one particular problem with using fluorophores is there inherent photo instability, where most fluorophores typically photo degrade after about 103 excitation/emission event cycles. Thus, one constraint in immunosensing is the detectability of the fluorophore.
Notably, light scattering by metallic structures in known but heretofore several additional properties related to the light scattering from multiple metallic structures have been ill explored for biosensing applications. Thus, it would be advantageous to explore other scattering properties of nanoparticles and the interaction therebetween for affinity biosensing, including the spatial distribution of light scatter and its subsequent polarization dependence; and the ability of for noble metal nanostructures to dipole-couple over very large distances, thereby effectively breaking the fluorescence resonance energy transfer (FRET) limit imposed by current organic fluorophores.